Silvia Janietz - Academia.edu (original) (raw)

Papers by Silvia Janietz

Nature communications, Jan 23, 2018

The fact that organic solar cells perform efficiently despite the low dielectric constant of most... more The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.

Applied Physics Letters, 2015

The influence of ether groups in the side chain of spin coated regioregular polythiophene derivat... more The influence of ether groups in the side chain of spin coated regioregular polythiophene derivatives on the polymer layer formation and the hybrid solar cell properties was investigated using electrical, optical, and X-ray diffraction experiments. The polymer layers are of high crystallinity but the polymer with 3 ether groups in the side chain (P3TOT) did not show any vibrational fine structure in the UV-Vis spectrum. The presence of ether groups in the side chains leads to better adhesion resulting in thinner and more homogeneous polymer layers. This, in turn, enhances the electronic properties of the planar c-Si/poly-thiophene hybrid solar cell. We find that the power conversion efficiency increases with the number of ether groups in the side chains, and a maximum power conversion efficiency of η = 9.6% is achieved even in simple planar structures.

Journal of Physics: Condensed Matter, 2016

Au surfaces are functionalized by aminobenzene (AB) and 2-aminotoluene (AT) using the electrochem... more Au surfaces are functionalized by aminobenzene (AB) and 2-aminotoluene (AT) using the electrochemical reduction of diazotized 1,4-diaminobenzene and 2,5-diaminotoluene. The IR spectroscopic measurements reveal the successful modification of Au surfaces by AB and AT. Both types of layers show similar thicknesses as obtained by microgravimetric measurements via electrochemical quartz crystal microbalance (EQCM). However, the faradaic efficiency for the grafting of AT onto an EQCM-Au sensor was 6% compared to 41% for the grafting of AB. This behavior points to a steric hindrance during the binding of AT to the EQCM surface induced by the additional methyl group present in the toluene derivative. The AB and AT functionalized surfaces have been further modified by the amidation reaction of EDC/NHS activated 4-nitrobenzoic acid. This model system reveals that the amidation reaction is slightly hindered in case of the AT layer due to the presence of the methyl group close to the amino group. This behavior leads to a four times less amount of amide bonds at the AT compared to AB modified Au surfaces as obtained from IR spectroscopic measurements.

First International IEEE Conference on Polymers and Adhesives in Microelectronics and Photonics. Incorporating POLY, PEP & Adhesives in Electronics. Proceedings (Cat. No.01TH8592)

Polymer materials are well known in modern microelectronics. In the past, however, they have been... more Polymer materials are well known in modern microelectronics. In the past, however, they have been used mainly for passive materials, substrate and encapsulating materials and adhesives. Special polymers with conjugated units have been employed as functional materials for the last decade. The first displays based on organic light-emitting diodes (OLEDs) have been on the market for a short time and

Polytronic 2005 - 5th International Conference on Polymers and Adhesives in Microelectronics and Photonics

ABSTRACT

Macromolecular Chemistry and Physics

9th International Symposium on Electrets (ISE 9) Proceedings

The development of multilayer light-emitting diodes (LED) consisting of evaporated organic materi... more The development of multilayer light-emitting diodes (LED) consisting of evaporated organic materials by Tang and van Slyke [ 11 stimulated significant progress in the development of design concepts of multilayer structures as we11 as in molecular design of fluorescent dyes and charge transport dyes. Since the first discovery of polymer-based light emitting diodes in 1990 [2] the interest of the charge transport phenomena in such devices increased rapidly. The efficiency of the injection of electrons and holes plays an important role for the electroluminescence of LED's. The electronic nature of the organic materials determined the parameters for good electron and hole injection and transport capability.

Synthetic Metals, 1997

New polymers with 1,3$oxadiazole and thianthrene units in the main chain were synthesized and the... more New polymers with 1,3$oxadiazole and thianthrene units in the main chain were synthesized and the electrochemical doping processes of these polymers have been studied by cyclic voltammetric measurements. The investigated silicon containing poly-1,3,4oxadiazole can be reversibly reduced electrochemically. For the polymer with thianthrene units the cyclovoltammogram shows a reversible oxidation process. A smgle layer device with the thianthrene polymer was prepared by spin-coating technique.

Macromolecules, 2006

Recently, two different groups have reported independently that the mobility of field-effect tran... more Recently, two different groups have reported independently that the mobility of field-effect transistors made from regioregular poly(3-hexylthiophene) (P3HT) increases strongly with molecular weight. Two different models were presented: one proposing carrier trapping at grain boundaries and the second putting emphasis on the conformation and packing of the polymer chains in the thin layers for different molecular weights. Here, we present the results of detailed investigations of powders and thin films of deuterated P3HT fractions with different molecular weight. For powder samples, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) were used to investigate the structure and crystallization behavior of the polymers. The GPC investigations show that all weight fractions possess a rather broad molecular weight distribution. DSC measurements reveal a strong decrease of the crystallization temperature and, most important, a significant decrease of the degree of crystallinity with decreasing molecular weight. To study the structure of thin layers in lateral and vertical directions, both transmission electron microscopy (TEM) and X-ray grazing incidence diffraction (GID) were utilized. These methods show that thin layers of the low molecular weight fraction consist of well-defined crystalline domains embedded in a disordered matrix. We propose that the transport properties of layers prepared from fractions of poly(3-hexylthiophene) with different molecular weight are largely determined by the crystallinity of the samples and not by the perfection of the packing of the chains in the individual crystallites.

Macromolecular Chemistry and Physics, 2003

Polyquinolines are under discussion as n-type materials for organic field effect transistors, org... more Polyquinolines are under discussion as n-type materials for organic field effect transistors, organic linght-emitting diodes and organic solar cells together with other N-heterocycle-containing conjugated polymers. But most of these conjugated polyquinolines are not soluble in common organic solvents. Therefore, a new synthetic pathway is presented in this work leading to organo-soluble polyquinolines. The results of the synthesis and characterization of the obtained new organo-soluble polyquinolines are shown. The electrochemical reduction and oxidation behavior of the these polymers is studied by cyclovoltammetric measurements. All studied polymers can be reversibly oxidized and reduced. The absorption and luminesceence properties are investigated. Organic light-emitting diodes (OLEDs) are prepared from the new polyquinolines as an active single layer. The electroluminescence behavior and the characteristics of the OLEDs are discussed.

Macromolecular Chemistry and Physics, 2004

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Macromolecular Chemistry and Physics, 2002

ABSTRACT New symmetrical and unsymmetrical dialkoxy-substituted aromatic poly(1,3,4-oxadiazole)s ... more ABSTRACT New symmetrical and unsymmetrical dialkoxy-substituted aromatic poly(1,3,4-oxadiazole)s are investigated by cyclovoltammetric measurements. The electrochemical redox behaviour of these materials is compared with the chemical structure. The reduction behaviour allows the estimation of the electron affinity, and shows that these polymers are suitable candidates for electron-transport materials in electronic devices. Surprisingly it is found from UV-vis and fluorescence spectroscopic measurements that the polyoxadiazole with linear alkoxy side chains (C16) shows a reversible thermochromism in solution as well as in the solid state. The changes in this temperature range are ascribed to a phase transition between aggregated chains, in solution as well as in the film, at room temperature and molecularly dissolved polymer chains at higher temperature. The other poly(1,3,4-oxadiazole)s with branched alkoxy side chains exhibit an improved solubility and they are completely soluble in o-dichlorobenzene at room temperature. The absorption and emission behaviour does not change in the temperature range from 25°C to 100°C. The fluorescence excitation studies of these poly(1,3,4-oxadiazole)s show a blue emission in solution and in the film. Due to these results the poly(1,3,4-oxadiazole)s are used as electron transport and blue emitting layers in electronic devices like diodes or polymer light emitting diodes (PLED).

Macromolecular Chemistry and Physics, 1999

ABSTRACT Since the heterocyclic thianthrene is known to exhibit reversible oxidation behavior in ... more ABSTRACT Since the heterocyclic thianthrene is known to exhibit reversible oxidation behavior in cyclovoltammetric experiments, it may be used as charge transport material in electronic devices such as light emitting diodes (LEDs). In order to prepare transparent films for further electrochemical investigation and the setup of LED devices, new suitable thianthrene monomers were synthesized. Polymerization of these monomers yields four main-chain polymers with different chemical structure. Special emphasis is given to the preparation of poly(thianthrenylenevinylene) via the “precursor” route. All polymers exhibit excellent film casting properties and are characterized thoroughly.

Journal of the American Chemical Society, 2012

A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher... more A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC 70 BM compared to the well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open-circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphological studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates more strongly in pristine and blended layers, with a smaller amount of additives needed to achieve optimum device performance. Time-delayed collection field and charge extraction by linearly increasing voltage are used to gain insight into the effect of fluorination on the field dependence of free charge-carrier generation and recombination. F-PCPDTBT is shown to exhibit a significantly weaker field dependence of free charge-carrier generation combined with an overall larger amount of free charges, meaning that geminate recombination is greatly reduced. Additionally, a 3-fold reduction in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC 70 BM is largely determined by the field dependence of free-carrier generation, and this field dependence is considerably weaker compared to that of blends comprising the non-fluorinated polymer. For these optimized blends, a short-circuit current of 14 mA/cm 2 , an opencircuit voltage of 0.74 V, and a fill factor of 58% are achieved, giving a highest energy conversion efficiency of 6.16%. The superior device performance and the low band-gap render this new polymer highly promising for the construction of efficient polymerbased tandem solar cells.

Journal of Polymer Science Part A: Polymer Chemistry, 2010

A series of novel styrene derived monomers with triphenylamine-based units, and their polymers ha... more A series of novel styrene derived monomers with triphenylamine-based units, and their polymers have been synthesized and compared with the well-known structure of polymer of N,N 0-bis(3-methylphenyl)-N,N 0-diphenylbenzidine with respect to their hole-transporting behavior in phosphorescent polymer light-emitting diodes (PLEDs). A vinyltriphenylamine structure was selected as a basic unit, functionalized at the para positions with the following side groups: diphenylamine, 3-methylphenyl-aniline, 1-and 2-naphthylamine, carbazole, and phenothiazine. The polymers are used in PLEDs as host polymers for blend systems with the following device configuration: glass/indium-tin-oxide/PEDOT:PSS/polymer-blend/CsF/Ca/ Ag. In addition to the hole-transporting host polymer, the polymer blend includes a phosphorescent dopant [Ir(Me-ppy) 3 ] and an electron-transporting molecule (2-(4-biphenyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole). We demonstrate that two polymers are excellent hole-transporting matrix materials for these blend systems because of their good overall electroluminescent performances and their comparatively high glass transition temperatures. For the carbazole-substituted polymer (T g ¼ 246 C), a luminous efficiency of 35 cd A À1 and a brightness of 6700 cd m À2 at 10 V is accessible. The phenothiazine-functionalized polymer (T g ¼ 220 C) shows nearly the same outstanding PLED behavior. Hence, both these polymers outperform the well-known polymer of N,N 0-bis(3-methylphenyl)-N,N 0diphenylbenzidine, showing only a luminous efficiency of 7.9 cd A À1 and a brightness of 2500 cd m À2 (10 V). V